M9: install.sh, backup timer and docs for a public figcraft-server

packaging/server/install.sh sets up Debian/Ubuntu behind Caddy: binary from
its release archive, the source tree it sits in, the latest release or a
clone (temporary swap under 2 GB of memory), user, dirs and config only when
missing, systemd unit, Caddy site, nightly backup timer, ufw for 22/80/443;
safe to re-run for upgrades. docs/server.md gains Running a public server
(limits, expiry, moderation, operator responsibility, no end-to-end
encryption, threat model) and an install.sh walk-through; protocol, MCP,
CLI and parity docs updated.

.github/workflows/release.yml

@@ -284,7 +284,7 @@ jobs:
       - name: Check the archive
         run: |
           tar -tzf "dist/release/figcraft-server-$FIGCRAFT_VERSION-linux-${{ matrix.arch }}.tar.gz" | tee "$RUNNER_TEMP/server-contents.txt"
-          for f in '/figcraft-server$' '/figcraft-server\.service$' '/Caddyfile$' '/nginx\.conf$' '/server\.md$' '/LICENSE-MIT$'; do
+          for f in '/figcraft-server$' '/figcraft-server\.service$' '/Caddyfile$' '/nginx\.conf$' '/install\.sh$' '/figcraft-backup\.timer$' '/server\.md$' '/LICENSE-MIT$'; do
             grep -E "$f" "$RUNNER_TEMP/server-contents.txt"
           done
           ldd target/release/figcraft-server